OBSERVATIONS OF 2.3-MU-M CO BANDS IN 47-TUC GIANTS

As the cause of the CN variations seen in 47 Tuc stars (and in many other moderate-metallicity globular clusters) remains poorly understood, the 2.3 μm CO bands of four 47 Tuc red giants have been observed with the AAT Fabry-Perot Infrared Grating Spectrometer (FIGS), with the goal of deriving carbon abundances as well as 12C/13C ratios. All four stars have 3.42 < V - K < 3.52 (i.e., Teff = 3950 ± 70 K), yet show substantial differences in their DDO C(41 - 42) colors (Norris, Freeman, and Da Costa 1984), implying large variations in CN band strengths. Their CO colors (Frogel, Persson, and Cohen 1981) also reveal an anticorrelation with CN band strength. The 13CO lines at 2.345 μm are seen in our spectra of all four stars, although they are slightly stronger in the CN-strong/CO-weak stars. Simultaneous fits for [C/A] and 12C/13C in the observed spectra indicate that [C/A] is indeed lowest in the CN-strong stars. The low 12C/13C ratio in the CN-normal stars is rather surprising, as it has been suggested that subgiant stars in 47 Tuc with normal CN bands have not undergone substantial mixing of CN-processed material (Briley et al. 1989). However, the results found for these giants (3 < 12C/13C < 13) are consistent with other determinations of 12C/13C in Population II field and globular cluster giants.